1. Field of the Invention
The present invention relates to an apparatus for optical power transmission suitable for an application in an optically powered system in which a driving power of an electronic device such as a sensor is supplied from a distant driver section, and an output signal of the electronic device is transmitted to the driver section, both optically.
2. Description of the Background Art
In order to prevent the introduction of noises through a power supply and signal line, a small electronic device is used, usually of the battery driven optical transmission type which utilizes a chemical battery such as lithium cells or a solar battery such as photo cells.
Now, the use of a chemical battery necessitates regular replacement of the lithium cells, whereas, as shown in FIG. 1, the use of a solar battery in a small electronic device 3 requires a number of photo cells 1 to be connected in series in order to obtain a sufficient loading voltage with respect to a load 2. This is because a single photo cell is capable of generating only about 0.5 V, which alone is insufficient for driving the load 2. In addition, with a solar battery, the whole electronic device 3 is required to have a large enough size to accommodate necessary photosensitive areas.
As a solution, there has been proposed an optically powered system in which electric power is first converted into an optical power at a transmitting side by means of a light emitting element such as a laser diode, the converted optical power is then transmitted to a receiving side through an optical path such as an optical fiber or an optical wave guide, and the received optical power is converted back to the electric power to be loaded on the load at the receiving side by means of photo cells.
An example of a conventional optically powered system is shown in FIG. 2, in which light from a light emitting element 11 driven by a power source 9 of a driving section 10 is transmitted to a signal processing section 20 through an optical fiber 12, and converted into electric power by photo cells 13 of the signal processing section 20. The converted electric power is then utilized to drive a sensor 14. An output signal of the sensor 14 is processed at a processor 15, and the processed sensor output signal is converted into light signal at an E/O converter 16 and transmitted through another optical fiber 17 back to the driving section 10. The received sensor output signal in a form of light signal is then re-converted into an electric sensor output signal at an O/E converter 18 of the driving section 10.
In such an optically powered system, an introduction of noises through a power supply line can be prevented without need to ensure sufficient light exposure or large enough photosensitive areas, so that it becomes possible to make the system compact.
However, such an optically powered system still requires a number of photo cells to be connected in series, because as mentioned earlier a single photo cell is capable of generating only about 0.5 V.
Furthermore, in order to make each photo cell generate power at high efficiency it is necessary to divide the light coming from the optical path into a number of equal parts. This in turn requires a light dividing device such as a diffraction grating 25 shown in FIG. 3, or an optical star coupler 26 shown in FIG. 4, but an incorporation of such a light dividing device inevitably makes the system complicated.
Meanwhile, there is also a problem concerning a matching of load impedance of the photo cells and the impedance required by the load.
Namely, as shown in FIG. 5, the power extractable from a photo cell is determined from an area on a V-I plot enclosed by V and I axes and lines parallel to V and I axes passing through an intersection P of a V-I characteristic curve 27 and a load characteristic curve 28, which appears shaded in FIG. 5. Since these characteristic curves 27 and 28 are different for the different amounts of incident light, there is a unique set of the amount of incident light and the load impedance for a given amount of power for which the maximum efficiency is obtainable, i.e., the area of shaded region in FIG. 5 is largest. The maximum efficiency is achieved by a load impedance of the photo cell of approximately 100.OMEGA. for incident light beams of 1 to 10 mW.
On the other hand, the small electronic device of concern here is a micropower system requiring less than 1 mW of power, which corresponds to an impedance of approximately 10K.OMEGA. for driving voltage of 2.0 V.
Thus, even when a number of photo cells are connected in series, the maximally efficient load impedance for the photo cells are far short of the impedance required by the load, so that the efficient extraction of power from the photo cells has been impossible.
In this regard, as mentioned earlier, a single photo cell is capable of generating only about 0.5 V. Use of amplifier circuitry to solve this problem is prohibited, since there is no amplifier circuitry which can operate at such a low voltage.
For this reason, in optically powered systems, the light to electricity conversion efficiency has been low.
In addition, there is a considerable power loss due to dissipation in the optical path, so that it has been necessary for the transmitting side to supply an amount of power which is a number of times greater than that required at the receiving side.
In the optically powered system, such as shown in FIG. 2, semiconductor lasers are usually employed as light sources, and it is well known that the lifetime of a semiconductor laser is inversely proportional to a square to fourth power of a light emission power, so that as the light emission power increases as in the optically powered system, the lifetime of the semiconductor laser shortens exponentially.
Moreover, a conventional optically powered system is required to use larger than otherwise necessary amount of power in order to be able to deal with fluctuations in the light emission efficiency of the semiconductor lasers, fluctuation in the conversion efficiency of the photo cells due to environmental conditions, and the greatest amount of power loss in the optical path occurring for the longest optical path to be used.
Therefore, the lifetime of the semiconductor lasers in the optically powered system has been made unnecessarily shorter than that inevitably required by the operation of the system alone.